Skinning rigged model - Fine adjustments in Weight Painting for twisted skin
You can subtract weight in small amounts, but it will still affect a large area, because the weight is per-vertex.

To change how much you add/subtract/etc., change the Strength slider in the left (T) panel.

As for exactly how much weight you need, I can't help you there. (:

(With your particular model, it's so low-poly I might just tweak the weights manually, from the Vertex Weights section in the right (N) panel.)